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I awoke far too early this morning and to pass the time I was perusing threads I normally don't; this time it was about approach speeds in an Archer. Now last time I was in a Archer was a CFI sometime in the early '80's so I really don't remember much about it, but one thing did catch my attention was the mention of using the same approach speed in either a calm or steady headwind approach. Now this has nothing to do with the Archer specifically, it applies to all aircraft, but is especially applicable to heavy ones or those without power, such as gliders and emergency procedures.
We all learned in primary training the velocity of the wind typically decrease the closer we get to the surface, surface friction, etc. Nothing new there, but how does this relate to landing? Simple, the wind component at pattern altitude is most likely higher than what is reported on the surface, be it headwind, crosswind, or tailwind as we fly around the pattern. Especially important is what it does as we descend down the final approach path. As counterintuitive as it may seem to some, a decreasing headwind on final will lead to the aircraft sinking below the glide path and coming up short of the intended touchdown point. Think the dreaded downwind turn...
Now, if this all seem a bit confusing, here is why. Say for example the difference in headwind between 500' AGL and the surface is 10 kts, (5 on the surface and 15 kts as you turn final) and you fly a constant 70 kts. At the turn to final your GS will be 55 kts, at round-out it will be 65 kts, 10 kts faster. So where did the energy to accelerate the mass of the airplane 10 kts come from? Unless you add power the only place it can come from is your altitude, by converting potential energy into kinetic. How many time has someone said there is a "sinker" on final? It probably wasn't a descending air mass that got their attention, it was the effect of the decreasing headwind component as they descended thru the wind gradient on final. Some airports do have a propensity for sinking air on final, Sedona, AZ and Whitehorse, YT come to mind, but they are the exception.
So what does this all mean? It means that if there is a change in wind headwind component on the final approach and you fly a constant airspeed be prepared to do a little throttle jockeying, especially in the last 50-100', to keep things in order. Not the best recipe for a smooth arrival. A easier way to deal with it is to note the ground speed on your GPS at the top of your final approach, if it indicts a notably stronger headwind component than what is being reported at the surface you can bet you are going to find one of these "sinkers" toward the bottom. An easy fix is to simply add the difference in the headwind component between where you presently are and the surface to your normal approach speed and proceed normally, you will find that with the power setting you would normal use the additional speed will magically be gone by the time you reach the runway with little additional changes in power.
Jeff
We all learned in primary training the velocity of the wind typically decrease the closer we get to the surface, surface friction, etc. Nothing new there, but how does this relate to landing? Simple, the wind component at pattern altitude is most likely higher than what is reported on the surface, be it headwind, crosswind, or tailwind as we fly around the pattern. Especially important is what it does as we descend down the final approach path. As counterintuitive as it may seem to some, a decreasing headwind on final will lead to the aircraft sinking below the glide path and coming up short of the intended touchdown point. Think the dreaded downwind turn...
Now, if this all seem a bit confusing, here is why. Say for example the difference in headwind between 500' AGL and the surface is 10 kts, (5 on the surface and 15 kts as you turn final) and you fly a constant 70 kts. At the turn to final your GS will be 55 kts, at round-out it will be 65 kts, 10 kts faster. So where did the energy to accelerate the mass of the airplane 10 kts come from? Unless you add power the only place it can come from is your altitude, by converting potential energy into kinetic. How many time has someone said there is a "sinker" on final? It probably wasn't a descending air mass that got their attention, it was the effect of the decreasing headwind component as they descended thru the wind gradient on final. Some airports do have a propensity for sinking air on final, Sedona, AZ and Whitehorse, YT come to mind, but they are the exception.
So what does this all mean? It means that if there is a change in wind headwind component on the final approach and you fly a constant airspeed be prepared to do a little throttle jockeying, especially in the last 50-100', to keep things in order. Not the best recipe for a smooth arrival. A easier way to deal with it is to note the ground speed on your GPS at the top of your final approach, if it indicts a notably stronger headwind component than what is being reported at the surface you can bet you are going to find one of these "sinkers" toward the bottom. An easy fix is to simply add the difference in the headwind component between where you presently are and the surface to your normal approach speed and proceed normally, you will find that with the power setting you would normal use the additional speed will magically be gone by the time you reach the runway with little additional changes in power.
Jeff
